Brexit will have happened in 2 years. This is almost certainly going to be a bad thing for the UK. There was going to be an election the year after. So the Tories would be trying to get re-elected with the whole country reeling from the fallout of Brexit, weakening their position considerably. And who knows what the opposition will be like at that point?
Now, however, they're riding high in the polls, the Lib Dems haven't recovered from the Coalition, Labour is toothless and falling apart, and she can frame voting Tory as being the only way to ensure a good result of Brexit - meaning people are even more likely to vote for them. So they get elected now, and then there's not another election until 2022, 3 years after Brexit, giving the country time to recover and people enough time to forget the reason for the country being up the crapper in the first place.
Basically, doing it like this guarantees them control of the country for the next 5 years, and gives them the best chance of winning the next election, too.